Heart disease and its manifestations, including coronary artery disease, myocardial infarction, congestive heart failure and cardiac hypertrophy, clearly present a major health risk in the United States today. The cost to diagnose, treat and support patients suffering from these diseases is well into the billions of dollars. Two particularly severe manifestations of heart disease are myocardial infarction and cardiac hypertrophy. With respect to myocardial infarction, typically an acute thrombocytic coronary occlusion occurs in a coronary artery as a result of atherosclerosis and causes myocardial cell death. Because cardiomyocytes, the heart muscle cells, are terminally differentiated and generally incapable of cell division, they are generally replaced by scar tissue when they die during the course of an acute myocardial infarction. Scar tissue is not contractile, fails to contribute to cardiac function, and often plays a detrimental role in heart function by expanding during cardiac contraction, or by increasing the size and effective radius of the ventricle, for example, becoming hypertrophic.
With respect to cardiac hypertrophy, one theory regards this as a disease that resembles aberrant development and, as such, raises the question of whether developmental signals in the heart can contribute to hypertrophic disease. Cardiac hypertrophy is an adaptive response of the heart to virtually all forms of cardiac disease, including those arising from hypertension, mechanical load, myocardial infarction, cardiac arrhythmias, endocrine disorders, and genetic mutations in cardiac contractile protein genes. While the hypertrophic response is initially a compensatory mechanism that augments cardiac output, sustained hypertrophy can lead to dilated cardiomyopathy (DCM), heart failure, and sudden death. In the United States, approximately half a million individuals are diagnosed with heart failure each year, with a mortality rate approaching 50%. The causes and effects of cardiac hypertrophy have been extensively documented, but the underlying molecular mechanisms have not been elucidated. Understanding these mechanisms is a major concern in the prevention and treatment of cardiac disease and will be crucial as a therapeutic modality in designing new drugs that specifically target cardiac hypertrophy and cardiac heart failure.
Treatment with pharmacological agents still represents the primary mechanism for reducing or eliminating the manifestations of heart failure. Diuretics constitute the first line of treatment for mild-to-moderate heart failure. If diuretics are ineffective, vasodilatory agents, such as angiotensin converting enzyme (ACE) inhibitors (e.g., enalopril and lisinopril) or inotropic agent therapy (i.e., a drug that improves cardiac output by increasing the force of myocardial muscle contraction) may be used. Unfortunately, many of these standard therapies have numerous adverse effects and are contraindicated in some patients. Thus, the currently used pharmacological agents have severe shortcomings in particular patient populations. The availability of new, safe and effective agents would undoubtedly benefit patients who either cannot use the pharmacological modalities presently available, or who do not receive adequate relief from those modalities.
The adult heart is a dynamic organ capable of significant remodeling and hypertrophic growth as a means of adapting function to altered workloads or injury. Hemodynamic stress or neuroendocrine signaling associated with myocardial infarction, hypertension, aortic stenosis, and valvular dysfunction evoke a pathologic remodeling response through the activation of intracellular signaling pathways and transcriptional mediators in cardiac myocytes. Activation of these molecular pathways enhances cardiomyocyte size and protein synthesis, induces the assembly of sarcomeres, and causes reexpression of fetal cardiac genes. Although aspects of the hypertrophic response after acute and chronic stress may initially augment cardiac output, prolonged hypertrophy is a major predictor of heart failure and sudden death. There have been major advances in the identification of genes and signaling pathways involved in this disease process, but the overall complexity of hypertrophic remodeling suggests that additional regulatory mechanisms remain to be identified.
MicroRNAs (miRNAs or miRs) have recently been implicated in a number of biological processes including regulation of developmental timing, apoptosis, fat metabolism, and hematopoietic cell differentiation among others. MiRNAs are small, non-protein coding RNAs of about 18 to about 25 nucleotides in length that regulate gene expression in a sequence-specific manner. MiRNAs act as repressors of target mRNAs by promoting their degradation, when their sequences are perfectly complementary, or by inhibiting translation, when their sequences contain mismatches.
MiRNAs are transcribed by RNA polymerase II (pol II) or RNA polymerase III (pol III; see Qi et al. (2006) Cellular & Molecular Immunology Vol. 3:411-419) and arise from initial transcripts, termed primary miRNA transcripts (pri-miRNAs), that are generally several thousand bases long and are derived from individual miRNA genes, from introns of protein coding genes, or from poly-cistronic transcripts that often encode multiple, closely related miRNAs. See review of Carrington et al. (2003). Pri-miRNAs are processed in the nucleus by the RNase Drosha into about 70- to about 100-nucleotide hairpin-shaped precursors (pre-miRNAs). Following transport to the cytoplasm, the hairpin pre-miRNA is further processed by Dicer to produce a double-stranded miRNA (Lee et al., 1993). The mature miRNA strand is then incorporated into the RNA-induced silencing complex (RISC), where it associates with its target mRNAs by base-pair complementarity. In the relatively rare cases in which a miRNA base pairs perfectly with an mRNA target, it promotes mRNA degradation. More commonly, miRNAs form imperfect heteroduplexes with target mRNAs, affecting either mRNA stability or inhibiting mRNA translation.
The 5′ portion of a miRNA spanning bases 2-8, termed the ‘seed’ region, is especially important for target recognition (Krenz and Robbins, 2004; Kiriazis and Kranias, 2000). The sequence of the seed, together with phylogenetic conservation of the target sequence, forms the basis for many current target prediction models. Although increasingly sophisticated computational approaches to predict miRNAs and their targets are becoming available, target prediction remains a major challenge and requires experimental validation. Ascribing the functions of miRNAs to the regulation of specific mRNA targets is further complicated by the ability of individual miRNAs to base pair with hundreds of potential high and low affinity mRNA targets and by the targeting of multiple miRNAs to individual mRNAs.
The high sequence conservation of many miRNAs across metazoan species suggests strong evolutionary pressure and participation in essential biologic processes (Reinhart et al., 2000; Stark et al., 2005). Indeed, miRNAs have been shown to play fundamental roles in diverse biological and pathological processes, including cell proliferation, differentiation, apoptosis, and carcinogenesis in species ranging from Caenorhabditis elegans and Drosophila melanogaster to humans. However, there remains limited information on the role that miRNAs play in cardiogenesis and molecular events that can contribute to heart disease.